Sources, seasonality, and trends of southeast US aerosol: an integrated analysis of surface, aircraft, and satellite observations with the GEOS-Chem chemical transport model

摘要

We use an ensemble of surface (EPA CSN, IMPROVE, SEARCH, AERONET), aircraft(SEACRS), and satellite (MODIS, MISR) observations over the southeastUS during the summer–fall of 2013 to better understand aerosol sources in theregion and the relationship between surface particulate matter (PM) andaerosol optical depth (AOD). The GEOS-Chem global chemical transport model(CTM) with 25 × 25 km resolution over North America is usedas a common platform to interpret measurements of different aerosol variablesmade at different times and locations. Sulfate and organic aerosol (OA) arethe main contributors to surface PM (mass concentration of PM finerthan 2.5 μm aerodynamic diameter) and AOD over the southeast US. OAis simulated successfully with a simple parameterization, assumingirreversible uptake of low-volatility products of hydrocarbon oxidation.Biogenic isoprene and monoterpenes account for 60 % of OA, anthropogenicsources for 30 %, and open fires for 10 %. 60 % of total aerosolmass is in the mixed layer below 1.5 km, 25 % in the cloud convectivelayer at 1.5–3 km, and 15 % in the free troposphere above 3 km. Thisvertical profile is well captured by GEOS-Chem, arguing against ahigh-altitude source of OA. The extent of sulfate neutralization( = [NH]/(2[SO] + [NO]) is only0.5–0.7 mol mol in the observations, despite an excess of ammoniapresent, which could reflect suppression of ammonia uptake by OA. This wouldexplain the long-term decline of ammonium aerosol in the southeast US,paralleling that of sulfate. The vertical profile of aerosol extinction overthe southeast US follows closely that of aerosol mass. GEOS-Chem reproducesobserved total column aerosol mass over the southeast US within 6 %,column aerosol extinction within 16 %, and space-based AOD within8–28 % (consistently biased low). The large AOD decline observed fromsummer to winter is driven by sharp declines in both sulfate and OA fromAugust to October. These declines are due to shutdowns in both biogenicemissions and UV-driven photochemistry. Surface PM shows far lesssummer-to-winter decrease than AOD and we attribute this in part to theoffsetting effect of weaker boundary layer ventilation. The SEAC4RS aircraftdata demonstrate that AODs measured from space are consistent with surfacePM. This implies that satellites can be used reliably to infersurface PM over monthly timescales if a good CTM representation ofthe aerosol vertical profile is available.